GEM—Geologic Emissions of Methane, the missing source in the atmospheric methane budget

Abstract

Central to any study of climate change is the development of an inventory that identifies and quantifies natural and anthropogenic sources and sinks of greenhouse gases (GHG). Recent studies have demonstrated that geologic emissions of methane (GEM), although not considered in the inventories of the IntergovernmentalPanel on Climate Change (IPCC), are an important GHG source. Etiope and Klusman (2002, Chemosphere 49, 777–789) documented that significant amounts of methane, produced within the Earth crust, are released naturally into the atmosphere through faults and fractured rocks. Major GEMs are related to hydrocarbon production in sedimentary basins (biogenic and thermogenic methane), through continuous exhalation and eruptions from more than 1200 onshore and offshore mud volcanoes (MVs), through diffuse soil microseepage, and shallow marine seeps; secondarily, methane is released from geothermal and volcano-magmatic systems. Minor geologic sources are those related to natural exhalation from coal-bearing rocks (influenced by mining activities), degassing from crystalline basement and mantle. While marine seeps have been studied for decades, methane flux from MVs has been the object of detailed measurements only since 2001, when hundreds of gas flux measurements were performed from vents and soilin the main terrestrial MVs of Europe, in Romania and Italy (Etiope et al.,2003, Geophysical Research Letters 30, 1094, doi:10.1029/2002GL016287; and references therein). In 2003 gas flux was measured in Azerbaijan, which hosts the world’s biggest MVs and densest MV population (Etiope et al., 2004, Geology, in press). In all areas investigated around 102–103 tons of methane per km2 are annually injected into the atmosphere. The global estimates of GEM from MVs range from 5 to 13Tg yr-1 (Etiope and Milkov, 2004, Environmental Geology, in press).